1. Field of the Invention
The present invention relates to white light-emitting devices which output white light suitable for illumination. More particularly, the invention relates to white light-emitting semiconductor devices which are equipped with phosphors as light-emitting factors and with a light-emitting semiconductor element as an excitation source for the phosphors.
In the invention and in this description, light having a color which has deviations Duv from the black-body radiation locus in the range of −20 to +20 is called white light. The definition of Duv (=1,000 duv) is in accordance with JIS Z8725:1999 “Method for Determining Distribution Temperature and Color Temperature or Correlated Color Temperature of Light Sources”.
2. Description of the Related Art
White LEDs configured so as to output white light on the basis of a combination of a gallium nitride-based light-emitting diode (LED) element and a phosphor have recently come to be used also as a light source of an illumination.
In illumination applications, there is a demand for white LEDs having a color temperature of 3500 K or lower (Patent Document 1). A major factor that has made it possible to produce such low color temperature white LEDs, which exhibit also high luminance that allows the white LEDs to be used in illumination, has been the development of alkaline earth siliconitride-based high-luminance red phosphors that use Eu2+ as an activator, for instance, (Ca,Sr,Ba,Mg)AlSiN3:Eu, (CaAlSiN3)1-x(Si(3n+2)/4NnO)x:Eu or the like (Patent Documents 2 to 4). These red phosphors are wide-band red phosphor which has a broad emission spectrum exhibiting full width at half maximum ranging from 80 to 120 nm. Thus, white LEDs having an average color rendering index (CRI) in excess of 80 can be produced by combining a blue LED element, as an excitation source and a generation source of blue light, and using a green phosphor as a generation source of green light.
By contrast, no thorough studies have been conducted thus far with the aim of improving the special color rendering index R9 of white LEDs. Herein, R9 is an index of reproducibility relating to high-saturation red. There is a strong demand for improved R9 in so-called warm white LEDs, which are white LEDs having a color temperature ranging from 2500 to 3500 K.
Patent Document 5 sets forth an example of the production of a white LED having a color temperature of 3800 K and R9=83, through combination of specific blue phosphors, green phosphors and red phosphors with a purple LED element.    Patent Document 1: JP-A-2004-103443    Patent Document 2: JP-A-2006-8721    Patent Document 3: JP-A-2008-7751    Patent Document 4: JP-A-2007-231245    Patent Document 5: JP-A-2006-49799    Patent Document 6: International Publication No. 2007-105631, pamphlet    Patent Document 7: International Publication No. 2009-072043, pamphlet    Patent Document 8: JP-A-2008-150549    Non-Patent Document 1: G. Chen, et al., Phys. Stat. Sol. (a) 205, No. 5, 1086-1092 (2008)
Various reports have been forthcoming on the production of white LEDs that exhibit high special color rendering index R9, by virtue of a combination of specific phosphors and a light-emitting semiconductor element. In most such reported cases, the phosphors are specified based on their formulas and emission peak wavelengths. However, it is a well-known fact that, even in phosphors that are represented by the same formula, dissimilar activator concentrations and dissimilar composition ratios of the elements that make up a base crystal result in dissimilar emission characteristics (peak wavelength of the emission spectrum, full width at half maximum and so forth). The inventors have also found, through experimentation, that the R9 value of white LEDs is not determined only by the emission peak wavelength of the phosphors that are used. Accordingly, it may be said that no design guidelines for white LEDs having high R9 values have been available thus far; what is more, reproducing the high R9 values of the reported white LEDs is far from easy.